Glossary

GLOSSARY

See visual acuity.

The ability of the visual organ to adjust its sensitivity and

function to the prevailing light level and color. The term can be

used for the process itself or for the final state. The retina is

said to be light adapted (corresponding to photopic vision) or

dark-adapted (scotopic vision). The size of the pupil plays only

a minor role in adaptation.

When two or more colored lights are added (overlaid), either on a

screen or otherwise superimposed on the retina.

The inability to perceive objects through otherwise normally

functioning sensory pathways (e.g. depth agnosia, movement

agnosia, color agnosia, prosopagnosia). Loss of knowledge.

Cells that convey information laterally to bipolar and ganglion

cell terminals in the inner plexiform layer of the retina.

Reduced vision without clear signs of a disease. May occur in

children who squint. Should be treated at a young age to

prevent permanent loss of coordinated vision of the two eyes

(e.g. depth vision).

Age-related macular degeneration is a disease that usually attacks

the receptors in or near the fovea and therefore typically has a

severe impact on visual resolution and contrast sensitivity. This

is the most common cause of low vision in elderly people in the

western world.

! ¼ 360f ¼ 2 = .

For a magnifying glass M ¼ 25=f ¼ 0:25P when the object is at a

distance equal to the focal length, f, from the lens and the

image is seen at infinity. P is the lens power in diopters.

Ring-shaped visual stimulus.

The fluid-filled space between cornea and iris/lens. With age, the

depth of the anterior chamber is reduced as the lens grows

larger.

A fluid produced by the ciliary body that serves to eliminate

waste products from the eye. It passes from the posterior

chamber to the anterior chamber before flowing out through

Schlemm’s canal.

Deviation of the curvature of the cornea from a spherical form

towards a cylindrical form. This causes the image planes for

some contours (e.g. vertical and horizontal contours) to be

different. Can be optically corrected.

Because of the distance between the two eyes, the left and

right eye will see the same detail on a three-dimensional

object at a different angle, and thus somewhat displaced

relative to the other details that are behind or in front of the

one in focus. The texture of an autostereogram contains two

separate images of the same three-dimensional object, each

as the object would appear for one eye alone. The spatial

relations of object details in each image are the same as

when the scene is viewed with one eye. By squinting, the

images are brought into register with each other, and the

sensory impression of depth arises.

GLOSSARY

The outward going nerve fiber from a cell. An axon transmits

signals to presynaptic terminals where the cell has established

synaptic contacts with other nerve cells.

Range of wavelengths (or frequencies) represented in a stimulus,

or to which a system is sensitive.

The hue of a color stimulus of constant chromaticity changes

when luminance changes.

The problem of how different attributes, such as color, move-

ment, shape, depth, etc., can be simultaneously attached to

an object. The processing of these attributes takes place in

different, separated brain regions and it takes time for

signals to travel between the areas.

The part of the frontal visual field that is viewed by both eyes.

Coordinated vision with both eyes.

Cells in the retina that convey information from the photo-

receptors to the ganglion cells.

The ability of some cortical blind people to point to the location

of light that they cannot see.

A spot in the monocular visual field that is blind because it

corresponds to the position on the retina where the nerve fibers

leave the eye, and where there are no photoreceptors.

Colors that arise at black and white borders due to dispersion, e.g.

by a prism or an imaging lens. When the beam of light is too

wide to produce a spectrum of monochromatic light upon

refraction, a partial spectrum of optimal colors may still be

seen at black and white borders.

Apparent amount of light emitted by a surface.

Brightness system.

An ailment due to opacities of the optic media of the eye. The eye

lens becomes less clear and the imaging on the retina more

diffuse. Removing the eye lens and replacing it with an

artificial lens is now a routine operation in this situation.

Commission Internationale de l’Eclairage.

Cyclic guanosine monophosphate. A substance that conveys

information to the cell membrane about light absorption in the

pigment disks of the receptors. CGMP controls the ion current

accross the cell membrane by opening special ion channels

(cGMP-gated channels) in darkness and closing them

in light.

Dispersion in the optical media of the eye gives rise to chromatic

aberrations. The eyes focus for wavelengths around 580 nm,

and they are therefore nearsighted (myopic) for short-

GLOSSARY

The self-adjustment of the visual system to the color of the

prevailing illumination in such a way that object surfaces

appear to have the same color for all daylight phases and for

most artificial lights. The visual system works towards

neutralizing (in an unknown way) the effect of the color of the

illumination. For example, a white surface appears white even

if the illumination changes from bluish daylight to yellowish

incandenscent light. See color constancy.

Colors are divided into achromatic colors (black, grays and

white) and chromatic colors (yellow, red, blue, green and their

transitions).

Two-dimensional color coordinates (r, g) in a unit color triangle

R þ G þ B ¼ 1, or the (x, y)-coordinates in the CIE system for

color measurement, in a plane where the sum of tristimulus

values X þ Y þ Z ¼ 1.

An isoluminant chromatic color stimulus has a chrominance

proportional to its chromatic difference from an achromatic

color of the same luminance (as defined in a colorimetric color

space). Color stimuli can be described by chrominance and

luminance coordinates (or by chrominance and luminance ratio

for object colors). Achromatic color stimuli have zero

chrominance.

Can be used both for a physical color stimulus and for the

qualitative, subjective experience.

A tendency for objects and reflecting surfaces to not change their

color appearance much when the color of the illumination

changes, e.g. from daylight to incandescent light.

Pairs of colors that yield white in an additive color mixture.

Receptor cell in the retina that operates at photopic

light conditions.

Michelson contrast, CMich ¼ ðLmax LminÞ=ðLmax þ LminÞ, is

commonly used for periodic stimuli. Weber contrast,

CWeb ¼ ðL LbÞ=Lb ¼ L=Lb, where L stands for stimulus

luminance and Lb for background luminance. Combined cone

contrast, CLMS ¼ ½ð1=3ÞðCL2 þ CM2 þ CS2Þ&1/2, where CL, CM

and CS are the individual cone contrasts for the absorptions

(excitations) in L-, M- and S-cones.

The ratio T between the contrast of an image and the imaged

object; T ¼ (image contrast)/(object contrast). For all modes of

optical reproduction, increasing spatial frequency (see grat-

ings) implies a smaller T. In the retinal image, contrasts are

reduced by dispersion, diffraction, light scatter from the eye

media and lens aberrations.

At photopic levels, curves of contrast sensitivity (1=Cthreshold)

plotted as a function of spatial and temporal frequency, have an

inverse U-form. For all but the lowest frequencies, contrast

sensitivity increases when luminance level increases, giving

curves their inverse U-form. The higher the adaptation

luminance, the smaller the stimulus contrasts that can be

detected (increased contrast sensitivity), and the finer the

details that can be resolved (increasing acuity).

GLOSSARY

Contrast sensitivity and resolution decrease with retinal

eccentricity (distance from the fovea). With increasing

age, sensitivity decreases at middle and high spatial

frequencies.

According to one theory, the primary visual cortex

has several separate mechanisms or cell groups each of

which is sensitive to a narrow band of spatial frequencies

and orientations. It has been assumed that, at any location

in the retina, there are at least six such mechanisms,

whose sensitivities overlap along the spatial frequency

dimension.

Flicker sensitivity (1/Cthreshold) varies as a function of

temporal frequency, and depends on spatial frequency.

Spatial contrast sensitivity curves for chrominance [i.e. for

chromatic contrast detection (color discrimination) at equal

luminance] differ from the curves for luminance contrast.

Chrominance sensitivity increases steadily towards lower

spatial frequencies, and sensitivity decreases rapidly at

higher spatial frequencies, with a cut-off frequency between

10 and 20 cycles/deg, depending on the color combination.

Transparent frontal surface of the eye.

Points on the retinae of the two eyes that have the same angular

distance from the fovea. See horopter.

Highly convoluted outer layers of cells that surround the top and

sides of the brain.

Cells that are activated by luminance decrements; their firing rate

increases when luminance decreases (also called OFF-cells).

Response to a decrement in stimulus luminance (response to

negative contrast).

Portion of a cell specialized for receiving inputs from other cells

via a great number of contact points; the synapses.

Change in membrane potential towards amore positive value

inside the cell relative to the negative resting potential.

Color vision deficiency resulting from the absence of M-cones.

Deuteranopes cannot distinguish between red, yellow and

yellow-green, or between purple, white and green.

Monochromatic light waves that are generated at the rim of an

aperture (such as the pupil) give rise to a pattern of

superimposed waves at a distance from the pupil. Analogous to

waves on water, the amplitudes of these waves add and subtract

on the retina depending on their relative phase

(interference). In the eye diffraction limits resolution and

visual acuity.

Color vision defect caused by the absence of one of the three

cone types. Protanopes lack L-cones while deuteranopes lack

M-cones. Both are unable to distinguish reddish colors from

yellow, white and green, although with different nuances.

Tritanopes lack S-cones and do not distinguish between

yellow, white and blue.

A measure of the ability to discriminate between two

suprathreshold stimuli along a certain stimulus dimension.

GLOSSARY

See refractive power.

The pathway from the photoreceptors through bipolar cells to

ganglion cells. These signals can be modified laterally

(sideways) by horizontal and amacrine cells, i.e. from cells

that integrate information laterally in the retina.

The ability to identify an object or an image after distinguishing

it from the background. Discrimination usually requires a

larger contrast than detection. One talks about color dis-

crimination when one sees a qualitative difference between

two color stimuli.

The index of refraction, n, of a medium (e.g. glass, water) varies

with the wavelength of light, and the refraction at the boundary

between two translucent media (glass and air, for example) is

therefore wavelength-dependent. White light that is refracted

by a glass prism gives rise to a spectrum where the short

wavelengths are refracted more than the long ones. One

consequence is that images of white/black borders produced by

a simple lens have colored borders (see border colors and

spectrum).

In the direction towards the top of the head.

A cell with cone opponency at every point in its receptive field.

Such cells are activated by light within a particular spectral

range (color) impinging on the center of their receptive field as

well as by the complementary color in their receptive field

surround.

Darkness system; should not be confused with D-cells or

Decrement cells, although an older theory assumes that they

are the same.

We depend on different types of photoreceptors for vision in

daylight and at night. The cones are the receptors for daylight

vision (photopic vision) and the rods are the receptors for

vision at low light levels (scotopic vision).

Gross potential reflecting the electric activity of retinal cells.

Six particularly simple color qualities (approximate wavelength

in brackets): black, white, yellow (ca. 570 nm), blue (ca.

470 nm), green (ca. 500 nm) and red (approximately comple-

mentary wavelength 495 nm). Also called unique colors.

The linear effect of absorption of light quanta in photoreceptors, a

process giving rise to changes in the membrane potential

(polarization) of receptors. It also refers to an increased

activity of nerve cells as opposed to the decrease of activity

during inhibition.

The eye’s lens is elastic and can change its radius of curvature to

adjust its refractive power (accommodation). The range of

accommodation is 12–16 diopters before the age of 25, but less

than 2 diopters after the age of 50. Usually, the closest point of

fixation increases to over 1 m around 70 years of age. The eye

absorbs UV light and becomes more yellow (transmits less

blue light) and rigid with age.

GLOSSARY

This law states that the smallest sensory threshold differences,

r, are proportional to the Weber ratio, I/I, and that they

can be integrated (summated) to a final sensory difference, R.

This leads to Fechner’s law: R ¼ c logðI=It), where It is

the threshold intensity. The dB scale for loudness of sound is

based on this law.

The impression of temporal variation in luminance or color.

The inverse of the threshold contrast for the perception of flicker

in a temporally modulated stimulus.

Functional magnetic resonance imaging is a means of imaging

brain activity. The technique is based on recording differences

in regional blood flow and in the oxygen uptake of active brain

cells between baseline and stimulus conditions.

The point at which a lens focuses parallel light rays.

The foveal pit is an area of about 1.5 mm in diameter

(corresponding to ca 5 ) which is free of capillaries and where

the nerve cells are bent aside, leaving the receptors more

directly exposed to light. The fovea is specialized for resolving

fine detail.

A smaller part of the fovea of about 1 in diameter that does not

contain rods; also called the ‘foveola’.

The part of the retina (the back of the eye) that is visible when

looking into the eye with an ophthalmoscope.

The cells representing the final stage of neural processing in the

retina. They generate nerve pulses that are led to the lateral

geniculate nucleus (LGN; or corpus geniculatum laterale in

Latin) via the optic nerve.

Theory that emphasizes properties of figures that transcend their

components.

An eye disease. If the transport of fluid out of the eye is somehow

impeded, this results in elevated pressure inside the eye, which,

if maintained, may eventually lead to visual loss (because the

pressure on the optic nerve becomes too high).

A pattern of alternating dark and light bars (see sinusoidal

grating).

A lateral contrast that extends over large areas, as opposed to the

local effects in border contrast (see also Mach bands).

Cells in the outer plexiform layer that convey information across

the retina (laterally) and modulate signal transmission from the

receptors to the bipolar cells. Horizontal cells may play a role in

the formation of the receptive field surround and in adaptation.

An imaginary circle that can be drawn through a fixated point

(an object) and the nodal point (close to the entrance pupil)

of the two eyes. Every point on this horopter is imaged in

corresponding retinal points, i.e. in points that have the

GLOSSARY

The (illusory) enhancement of lightness contrast across contours

arising from a luminance difference between two adjacent

areas.

The central region of the retina that contains the fovea. The

yellow spot, ca. 2–3 mm in diameter, corresponding to

about 10 .

Classical receptive field: a small area of the retina that evokes a

response in a nerve cell (e.g. a ganglion cell) when stimulated.

If an excitation of the photoreceptors within the center of this

area leads to activation, as in I-cells, stimulation of the

surrounding receptors will usually result in inhibition (center-

surround structure of the receptive field). D-cells are activated

by light decrements in the receptive field center or increments

in the surround, and they are inhibited by center increments. In

the fovea the receptive field center of a ganglion cell collects

information from only one or a few cones. Global receptive

field: a larger retinal area, extending beyond the classical

receptive field surround, through which the response of a cell

A neural element that transforms one type of energy to another.

Most sensory cells respond selectively to specific physical

stimuli, such as pressure, light, temperature, etc. Photorecep-

tors are excited by the absorption of light quanta and transform

this energy into a change in the electric potential across the cell

The ratio, , between the light flux, , that is reflected from a

surface, and the flux, P, that impinges on it: ¼ =P. ( )

gives its spectral distribution.

Ratio between the speed of light, c, in vacuum and its speed, v, in

the medium in question; n ¼ c=v.

The appearance of chromatic stimuli undergoes changes in

lightness, hue and chromatic strength depending on the

luminance of their surroundings. Object or surface colors that

are viewed in a well-illuminated, natural environment are

examples of related colors. Since they are all darker than a

white surface, they have some degree of induced blackness.

Blackness is in itself a typical related color. Unrelated color

refers to the appearance of light sources, of bright colors

viewed in the dark, or of stimuli with higher luminance

than their surroundings (they are sometimes called void

colors); 99 percent of all natural colors we see during

the day are related colors. The luminance ratio between a

surface and white (the reflection factor) determines whether

its color is related or unrelated. See also Bezold–Bru¨cke

A light-absorbing molecule in the cone and rod photoreceptors

which, when bound to different opsins, makes different

pigments with different spectral sensitivities.

The light-absorbing pigment of the rods (visual purple).

Receptor cell in the retina that is active at low (scotopic)

GLOSSARY

Areas of cortex specialized for vision, with further subdivisions

into functional units and modules. Receives input from the

LGN via primary visual cortex, V1.

Electrical brain activity in a collection of nerve cells provided by

a visual stimulus; can be recorded by scalp electrodes.

The image that is perceived consciously (as opposed to the

physical image on the retina).

The eye is filled with a gel-like, transparent fluid (vitreous) that

provides an inner pressure and thus maintains the spherical

shape of the eye. Fine threads crossing the vitreous may break

and form thin, curled, moving shadows that eventually sink to

the bottom of the eye.

Mathematical relation between the Weber ratio at threshold and

the subjective impression of lightness, s; s ¼ c L/L. When

integrated this leads to Fechner’s law.

Weber ratio, or Weber contrast, is defined as CWeber ¼ L=L. For

a wide range of photopic values of background luminance, L,

one finds that this ratio is constant at threshold (Weber’s law).

At high luminance levels, contrast sensitivity, 1/CWeber at

threshold, is highest for fields larger than 120 or 0.2 (when

they have sharp contours). For sinusoidal gratings with graded

contours the sensitivity decreases for bars larger than 10

arcmin (e.g. for spatial frequencies below 3 cycles/deg). See

contrast sensitivity, Weber’s law and Fechner’s law.

This law states that the Weber ratio is constant at threshold. The law

is valid at photopic light levels from about 50 to 10 000 cd/m2.